JPH0353256B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inorganic pigment and a method for producing the same, and more particularly to a yellow pigment obtained from an inorganic material. More particularly, the present invention relates to yellow pigments that are stable at high temperatures.

As is known to those skilled in the art to which this invention relates, yellow inorganic pigments, with the exception of nickel titanate, are not suitable for processing into plastics due to their thermal instability at temperatures above 175°C. That is, yellow pigments such as iron oxide, lead chromate, and zinc chromate are unsuitable for yellowing plastics such as polyethylene, polypropylene, polyvinyl chloride, polycarbonate, and polyamide. Furthermore, organic pigments are approximately
It exhibits color deterioration at temperatures higher than 175°C and is therefore excluded from use in plastic processing. Thus, the need for yellow pigments that are stable at high temperatures is readily apparent.

As detailed below, the present invention provides such stable yellow pigments.

To the best of applicant's knowledge, the following patents are most relevant to determining patentability: U.S. Patent No. 2,904,395;
No. 3822210, No. 3887479, No. 4075029 and No. 3832455.

Perhaps the most relevant of the above patents is US Pat. No. 3,822,210. This patent discloses a method for making zinc ferrite (spinel) pigments, but
However, the method used in this patent is different from the method of the present invention and does not teach or suggest the present invention since different products are manufactured by the different method. US Patent No. 3822210
The product made by No. is an isotropic ferrite. They are made under conditions of temperature and alkali to metal salt molar ratio that differ from those taught by the present invention. The patent teaches that the product is formed only at conditions of temperature and alkali to metal salt molar ratio that are above the dashed curves in Figures 2A-2C of the patent. . Furthermore, the method of the same patent includes a step of heating while oxidizing the precipitate.

The products made according to the invention are acicular ferrites. This product is formed under different conditions of temperature and alkali to metal salt molar ratio than those disclosed in US Pat. No. 3,822,210. Furthermore, the method of the invention involves heating the precipitate after the oxidation has taken place.

According to the invention, the yellow pigment is provided by a metallic spinel selected from the group consisting of iron and magnesium, zinc and tin, and mixtures thereof.

The spinel of the present invention can be produced by reacting ferrous sulfate and metal nitrate with a basic solution. The reaction proceeds at 5-50°C, preferably at room temperature. The precipitate is then aerated and reheated to obtain the spinel of the invention.

The spinels of the present invention are temperature stable up to about 900°C.

The present invention provides a yellow pigment that is temperature stable up to about 900°C. As known to those skilled in the art, yellow iron oxide pigments are called "goethite." These pigments are believed to be hydrated oxides with a crystalline composition structure of alpha-FeOOH. The cause of color instability is the transformation of yellow alpha-FeOOH to red alpha-Fe ₂ O ₃ . The transformation of Alpha-FeOOH to Alpha-Fe ₂ O ₃ occurs over a wide temperature range starting from 175° C. and varying depending on the nature of the pigment and its particle size.

This transformation is a function of temperature and length of exposure to such temperature. Generally, those skilled in the art believe that in this transformation alpha-FeOOH dehydrates to alpha-Fe ₂ O ₃ . When this phase transformation was studied using differential thermal analysis, the transformation occurred at 265°C.
It can be seen that it is completed at ~277°C, but the transformation occurs at much lower temperatures.

The invention is based on the fact that phase transformations are not only related to dehydration, but also to crystal structure, as explained in more detail below. For example, all four types of hydrated iron oxides with the composition FeOOH have different colors depending on their crystal structure, and only alpha-FeOOH is yellow.

The present invention is to produce a stable yellow pigment by providing spinels of iron and various metals.
The various metals used in the present invention are selected from the group consisting of magnesium, zinc, tin and mixtures thereof.

Without wishing to be bound by any theory, it appears that the distribution of iron and other atoms within the spinel structure greatly inhibits and limits the mobility of the iron atoms. This limitation is further enhanced by electrostatic interactions between metal, iron, oxygen and hydroxyl groups in the pigment. This limited mobility results in a higher degree of thermal energy being required to effect the yellow to red color transformation of the iron oxide.

The spinel of the present invention can be prepared by reacting hydrated ferrous sulfate and hydrated metal nitrates or their equivalents in solution initially at acidic and then alkaline pH. The reaction preferably proceeds at room temperature, but other temperatures from 5 to 50°C can be used if desired, as will be understood by those skilled in the art. The precipitate thus obtained is then oxidized to promote the oxidation of ferrous iron.

To summarize the method of the present invention, the method of the present invention is a method for producing a pigment having good stability at high temperatures, which is characterized by having the following steps: (a) water, soluble iron salt and magnesium; When forming a first aqueous solution consisting of at least one other salt of a non-ferrous metal selected from the group consisting of zinc and tin, the proportion of the iron salt and the non-ferrous salt in the solution is determined by When the pH is adjusted so that precipitation occurs, the respective amounts of iron and other metals present give a spinel of the formula XFe ₂ O ₄ (where (b) forming a first solution in such a manner that a substantial amount of oxidizable precipitate is formed; (b) adding an alkali metal carbonate, bicarbonate, and hydroxide to the first solution; When mixing a second solution containing a soluble alkalizing compound selected from the group to form a reaction solution, the amount of the second solution used and the dilution state of the first and second solutions are selected such that upon mixing particulate precipitates of submicron size are formed and such precipitation results in substantially complete depletion of metal content with a valence greater than 1 in the liquid phase of the reaction liquid. A mixed precipitation step to form a precipitate; (c) an oxidation step to oxidize the precipitate at 15 to 35°C to form spinel in an aqueous solution; (d) an aqueous solution containing the spinel to about 75° to 100°C
and (e) recovering the spinel.

In the present invention, specific reference will be made to hydrated metal nitrates, in particular Mg(NO ₃ ) ₂ ·6H ₂ O and Zn(NO ₃ ) ₂ ·6H ₂ O, but other equivalent materials will be readily apparent to those skilled in the art. It will be understood by those skilled in the art that these methods can be used with desirable results. In general, it is preferred to use hydrated salts because they are otherwise equivalent or somewhat more soluble in water than their anhydrous counterparts. Once an aqueous solution of the desired strength is obtained, it makes no difference whether the salt was originally in a hydrated form or not.

Optionally, nitrate ions may be replaced by other chloride ions or various soluble anions such as sulfate ions. Also, as mentioned elsewhere, divalent tin can be used as the cation.

Suitable bases or solutions according to the invention are alkali metal bases, such as sodium, potassium, or lithium carbonates, hydroxides, bicarbonates, and the like. A particularly preferred base is sodium carbonate.
Usually, stoichiometric equivalents of base are used. More specifically, it means, for example, using 1 mole of sodium carbonate (or its equivalent, eg, 2 moles of sodium bicarbonate) for each mole of hydrated ferrous sulfate. For example, as far as the effect on ferrous sulfate is concerned, sodium carbonate seems to be involved in the reaction as shown in the following equation.

FeSO ₄ +H ₂ O + Na ₂ CO ₃ →Fe(OH) ₂ +Na ₂ SO ₄ +CO ₂ ↑ Also, as far as the effect on magnesium nitrate is concerned, sodium carbonate is thought to be involved in the reaction as follows.

Mg(NO ₃ ) ₂ +H ₂ O+Na ₂ CO ₃ →Mg(OH) ₂ +2NaNO ₃ +CO ₂ ↑ The use of stoichiometric equivalents of sodium carbonate, etc. is usually preferred, but the amount used may not be stoichiometric. In some cases, satisfactory results may be obtained regardless of the situation. By "satisfactory results" is meant that the reaction in an aqueous medium gives a substantial yield of finely divided spinel having the desired composition and good high temperature stability. If the amounts do not deviate too much from the stoichiometric amount, the reaction operation is wasteful with respect to the use of carbonate or metal salts, but considerable yields of the desired spinel can be obtained. However, it is necessary to use at least enough sodium carbonate to precipitate both iron and a substantial portion of other metals such as zinc.

Obtaining particles of desired particle size is an important consideration in practicing the present invention. The particle size depends in part on the degree of dilution of the reaction solution used; other things being equal, the more dilute the solution, the finer the particle size.

It is preferred that the oxidation be carried out in most cases at about room temperature, i.e. 15 DEG-35 DEG C., with aeration.

Oxidation can be performed in any suitable manner.
One method is aeration (spraying the suspension-containing solution through a perforated pipe). Another method is to blow air or a mixture of oxygen and an inert gas into the suspension-containing solution. Yet another method is to shake the suspension-containing solution in a container with air or a mixture of oxygen and an inert gas.
Yet another possibility is hydrogen peroxide or any other chemical oxidizing agent that provides oxygen but does not impart color to the solution (such as potassium permanganate or sodium dichromate). One method is to use a suitable chemical oxidizing agent.
The required oxidation effect cannot be obtained practically by leaving the solution exposed to air or an oxygen atmosphere. The completion of oxidation can be monitored by titration.

The solution is then heated to a temperature of about 75°C to about 100°C. This elevated temperature is desirably maintained for a sufficient period of time to "complete crystallization", ie, ensure growth of the desired crystalline form in the product. Monitoring the progress and confirmation of completion of growth of the desired crystalline form can be performed using X-ray diffraction tests.

The spinel is then recovered by conventional filtration and washing.

The spinel thus obtained has a very fine grain size. The spinel particles have substantially all of their largest dimensions below 1 micron;
In most cases the maximum dimension is also e.g. 0.1
It is even finer, on the order of microns. The performance of pigments containing spinel particles differs depending on whether or not they have particles of the above dimensions. Generally, when spinel is obtained by the fusion process, substantially larger particles are obtained than those obtained by the present invention. The fine particles made by the present invention provide a purer color than the larger particles made by fusion methods.

Further illustrating the temperature-stable spinel product made by the present invention, the particles provided by the above method are acicular in shape and have a length on the order of 0.01-0.13 microns as shown in the drawings. have Figure 1 corresponds to the product of Example 3 below, and Figure 2 is a photomicrograph of a similar product made using zinc sulfate as the starting material instead of zinc nitrate. FIG. 3 shows the effect of calcination on the product shown in FIG. 1, where the particles are no longer acicular.

As shown in the photomicrographs, the above method yields a product different from any obtained in the above-mentioned US patent. Although the particles in U.S. Pat. No. 2,904,395 are described as acicular, they have a length of 0.4 to 1.0 microns compared to the particle size of 0.01 to 0.13 microns as shown in Figures 1 and 2. It is something.

It was confirmed by an X-ray diffraction test that the material shown in FIGS. 1 and 2 is spinel.
These materials do not exhibit a characteristic endotherm in differential thermal analysis at temperatures of 260-270°C. Such an endotherm characterizes the phase transformation from yellow alpha-FeOOH to _{red Fe2O3} . This is because the substance in Figures 1 and 2 is spinel, and alpha
Means not FeOOH.

U.S. Pat. No. 3,832,455 does not provide any knowledge regarding the diameter and shape of the particles obtained, but the method requires a calcination step and is the third step.
In view of the figure, it does not appear that this would result in a product containing acicular particles. US Patent No.
No. 3887479 also uses high temperature,
It does not include testing regarding the morphology of the product particles. Further, the difference between the particles of US Pat. No. 3,822,210 and the particles of the present invention is clear by comparing the micrographs of FIGS. Taking into account the difference in scale between Figures 3 and 3, the difference is small.

The particulate spinel of the invention obtained by the above process produces the desired yellow color when incorporated as a pigment in plastics in proportions and methods known to those skilled in the art.

In carrying out the invention, the spinel is preferably obtained from iron and magnesium, zinc or mixtures thereof. Such spinel is about 900
Demonstrates temperature stability up to ℃. Particularly good results have been obtained with iron-zinc spinels, which are preferred.

Those skilled in the art are well aware of how to incorporate spinel pigments into plastics and the like, and the necessary proportions, methods and equipment need no particular explanation.

A comparison of the relationship between weight loss and temperature for a conventional yellow iron oxide pigment and the iron-zinc spinel of the present invention reveals that the spinel of the present invention has less water present as hydroxyl groups than the pure iron oxide pigment. This is shown in Table 1 below.

Table 1 % weight loss temperature °C Iron-zinc spinel iron oxide 100 2.7 1.9 250 6.2 7.2 350 7.7 13.7 400 8.1 14.0 The chemical composition of the spinel of the present invention generally has a structure
It corresponds to XFe ₂ O ₄ (X is a metal as defined above).

Mixtures of metals may also be utilized, as described above. However, electron neutrality in the crystal must be maintained.

The present invention will be explained below using specific examples, but these are for illustrative purposes only and are not intended to limit the present invention.

Example 1 27.8 g/(gp) was added to a suitable reaction solution equipped with a dropping device, a cooling device, an aeration device and a stirring device.
Solution 3 _{of FeSO4.7H2O} and ₁₇ gpMg( _NO3 ) _2.6H2O was added. 231gp while stirring
200ml _{of Na2CO3} solution was added. The temperature in the container at the time of carbonate addition was 19°C.

A precipitate formed in the flask while stirring.

Once the precipitation was complete, air was blown into the flask via a vent. Simultaneously with aeration, the oxidation state of ferrous sulfate was monitored by titration with potassium permanganate.

After the aeration was completed, the precipitated solution was heated to 90°C, maintained at the same temperature for 2 hours, and then cooled to room temperature. The precipitate was then collected by filtering the solution. The precipitate was then washed and dried. In this way a temperature-stable yellow pigment was obtained. That is, this pigment did not show the above-mentioned transformation from goethite to hematite when subjected to differential thermal analysis. Also, this pigment can be added to approx.
It showed no color change when tested at high temperatures up to 700°C.

Example 2 27.8 gp FeSO ₄ 7H ₂ O and 19.8 gp Zn
(NO ₃ ) ₂・6H ₂ O iron-metal solution is used, and 46
Example 1 was repeated with the addition of 100 g of Na ₂ CO ₃ solution. A temperature-stable yellow pigment was obtained.

Example 3 A solution was prepared by dissolving 30.4 kg of ferrous sulfate heptahydrate and 21.3 kg of Zn(NO ₃ ) _{2.6H 2} O in 567.8 kg of water maintained at 20°C. 73 in water to this solution
A solution of 16.8Kg of Na ₂ CO ₃ was added. The reaction solution was aerated with air at 84.9/min until maximum oxidation of ferrous iron occurred by titration of potassium permanganate. The reaction solution was then heated to 90°C, filtered, washed and dried. In this way a temperature-stable yellow pigment was obtained.

Example 4 Example 3 was repeated except that 20.9 Kg of zinc sulfate heptahydrate was used in place of the above amount of zinc nitrate hexahydrate. In this case too, a temperature-stable yellow pigment was obtained.

Example 5 A sample of each pigment of Examples 1-4 was heated in an oven at 280°C for 1/2 hour. Each sample remained yellow.

The sample was still yellow when heated to 800°C for 1/2 hour.

The term "transparent pigment" as used in the claims means a pigment which, when added to a vehicle, is capable of providing a coating that is substantially transparent to visible light. This means that the pigment particles have a size that is less than the wavelength of visible light.

Although some aspects of the invention have been described above, various changes or modifications can be made without departing from the spirit and scope of the invention.

[Brief explanation of drawings]

Figure 1 shows 160,000 zinc ferrite pigment particles produced by the method of the present invention using zinc nitrate as a starting material.
It is a micrograph of magnification. FIG. 2 is a micrograph of zinc ferrite pigment particles prepared according to the present invention using zinc sulfate as a starting material at the same magnification. FIG. 3 is a micrograph at the same magnification of the particles shown in FIG. 1, which were fired at 600° C. for 20 minutes.

Claims (1)

[Claims] 1. A spinel in the form of acicular particles having a length of 0.01 to 0.13 microns and composed of oxygen, iron and at least one other metal, the spinel having the formula: XFe ₂ O ₄ (X represents the above-mentioned other metal and is selected from magnesium, zinc, tin and mixtures thereof). 2. The spinel of claim 1, wherein the other metal is selected from the group consisting of magnesium, zinc, and mixtures thereof. 3. The spinel according to claim 1, wherein the other metal is magnesium. 4. The spinel according to claim 1, wherein the other metal is zinc.